This invention relates to a stereo decoder with a phase-locked loop for use in a stereo FM radio receiver and more particularly pertains to such a stereo decoder wherein the bandpass of the phase-locked loop is switched from a broad to a narrow range of frequencies as the operational status of the loop changes from a pilot-signal acquisition mode to a pilot-signal locked mode.
The standard composite-stereo-multiplexed signal at the output of an FM radio discriminator circuit includes a left plus right (L+R) singlesided baseband of audible frequencies, a pilot signal at 19 KHz, and a left minus right (L-R) baseband centered about a suppressed 38 KHz subcarrier. These standard FM-radio stereo signal features are illustrated in FIG. 1.
It is usual practice to obtain the separated left and right audio signals by means of a stereo decoder having a phase-locked loop oscillator 10 to supply the missing 38 KHz subcarrier and a double balance gating circuit 12 or other mixer as illustrated in FIG. 2. The composite stereo signal is presented at the input conductor 14. Typically, in such prior art stereo demodulators, an "upper pair" of switching transistors in the double balanced mixer operate fully switched; i.e., off and on effecting full left and right audio signal separation.
The frequency and phase of the 38 KHz oscillator 10 are locked and synchronized to those of the 19 KHz pilot signal in the composite stereo signal. The conventional phase-locked loop includes the voltage controlled oscillator VCO 16, the divider circuit 18, the phase comparator 20 and the low pass filter 22 having a bandpass of about 300 Hz. The output of the VCO 16 is a fixed amplitude 38 KHz square wave signal. Such a conventional system is described by Stockwell and Camenzind in their paper "An Inductorless Monolithic Stereo Decoder", International Solid State Circuits Conference (Feb. 19, 1970) pages 106-107. A lamp 24 may be energized only when the 19 KHz pilot of a radio signal is locked by the phase-locked loop demodulator. This is accomplished by providing an inverting amplifier 26 connected to the output of the VCO 16 that in turn feeds another divide-by-two frequency divider 28. The composite signal from conductor 14 and the output of the divide-by-two divider 28 are fed into a mixer, e.g. double balanced gating circuit 30 that serves as a phase comparator and provides a binary one or high voltage level output only when the VCO 16 is locked to the 19 KHz pilot. This is followed by a smoothing capacitor 32, a Schmitt trigger circuit 34 and the lamp 24 to indicate the presence of the pilot signal to the operator.
FM radio provides the best quality broadcast radio reception available today. It offers wide-band high fidelity stereo sound that is relatively free from static, fading and interference from other stations. However, multipath noise in FM receivers remains a vexing problem especially for FM receivers in vehicles. Multipath noise comes from interference within an FM receiver between two signals arriving at the receiver along two different length paths from the same broadcasting station. Multipath noise in a fast moving FM receiver may take the form of bursts of noise and/or distortion.
Some currently used blend-function circuits reduce the stereo separation as a function of signal-to-noise ratio and are capable of moderating multipath noise. However, stereo separation and thus sound quality are degraded.
The signal-to-noise ratio in an FM receiver may be improved according to the disclosure by Torick et al in U.S. Pat. No. 4,485,483 issued Nov. 27, 1984, by a system of FM broadcasting and receiving that is denoted herein as the FMX system. "FMX" is a TRADEMARK of Broadcast Technology Partners, Bloomfield Hills, Mich. It is said that FMX receivers can provide reduction of background and multipath noise without loss of stereo separation. However, multipath reception (e.g. directly and indirectly from one station) is known to cause problems under some circumstances in the FMX decoder that lead to degradation in the sound.
It is an object of this invention to provide an improved FM stereo decoder to reduce or eliminate the effects of multipath interference in standard FM and in FMX radio receivers.